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Tetrahydrobiopterin (BH₄, THB, trade name Kuvan) or sapropterin (INN) is a naturally occurring essential cofactor of the three aromatic amino acid hydroxylase enzymes, used in the degradation of amino acid phenylalanine and in the biosynthesis of the neurotransmitters serotonin (5-hydroxytryptamine, 5-HT), melatonin, dopamine, norepinephrine (noradrenaline),epinephrine (adrenaline), and is a cofactor for the production of nitric oxide (NO) by the nitric oxide synthases.

History

Tetrahydrobiopterin was discovered to play a role as an enzymatic cofactor. The first enzyme found to use tetrahydrobiopterin is phenylalanine hydroxylase (PAH).

Biosynthesis

Tetrahydrobiopterin is biosynthesized from guanosine triphosphate (GTP) by three chemical reactions mediated by the enzymes GTP cyclohydrolase I (GTPCH), 6-pyruvoyltetrahydropterin synthase (PTPS), and sepiapterin reductase (SR).

Functions

Tetrahydrobiopterin has the following responsibilities as a cofactor:

• Tryptophan hydroxylase (TPH) for the conversion of L-tryptophan (TRP) to 5-hydroxytryptophan (5-HTP)
• Phenylalanine hydroxylase (PAH) for conversion of L-phenylalanine (PHE) to L-tyrosine (TYR)
• Tyrosine hydroxylase (TH) for the conversion of L-tyrosine to L-DOPA (DOPA)
• Nitric oxide synthase (NOS) for conversion of a guanidino nitrogen of L-arginine (L-Arg) to nitric oxide (NO)
• Alkylglycerol monooxygenase (AGMO) for the conversion of 1-alkyl-sn-glycerol to 1-hydroxyalkyl-sn-glycerol

Tetrahydrobiopterin has multiple roles in human biochemistry. One is to convert amino acids such as phenylalanine, tyrosine, and tryptophan to precursors of dopamine and serotonin, the body's primary neurotransmitters). Due to its role in the conversion of L-tyrosine in to L-dopa, which is the precursor for dopamine, a deficiency in tetrahydrobiopterin can cause severe neurological issues unrelated to a toxic buildup of L-phenylalanine; dopamine is a vital neurotransmitter, and is the precursor of norepinephrine and epinephrine. Thus, a deficiency of BH4 can lead to systemic deficiencies of dopamine, norepinephrine, and epinephrine. In fact, one of the primary conditions that can result from GTPCH-related BH4 deficiency is dopamine-responsive dystonia;[4] currently, this condition is typically treated with carbidopa/levodopa, which directly restores dopamine levels within the brain.

BH4 also serves as a catalyst for the production of nitric oxide. Among other things, nitric oxide is involved in vasodilation, which improves systematic blood flow. The role of BH4 in this enzymatic process is so critical that some research points to a deficiency of BH4 – and thus, of nitric oxide – as being a core cause of the neurovascular dysfunction that is the hallmark of circulation-related diseases such as diabetes.

FROM DR. AMY YASKO

The BH4 Three-Legged Stool

The added ammonia that is generated due to the enhanced breakdown of methylation cycle intermediates will also burden the adjoining urea cycle, thereby depleting a key intermediate called BH4, which plays a critical role in regulating neurotransmitters and therefore mood. BH4 is needed for serotonin, dopamine, conversion of phenylalanine to tyrosine and language-related function. The A1298C mutation in the MTHFR gene may also impact levels of BH4.

The drawing of a three-legged stool can help you visualize how the body maintains adequate levels of BH4. One leg is for CBS upregulations. The second leg is for MTHFR A1298C, another key SNP on the methylation pathway, which you will learn more about later in this chapter. The third leg is chronic bacteria/ aluminum. Stable BH4 levels require all three legs.

CBS upregulations weaken one leg of the stool by using up BH4 faster than it can be supplied. The NOS mutation can also exacerbate the CBS ammonia problem. In the adjacent urea cycle, inefficient NOS activity can lead to elevated ammonia levels, further draining BH4 limited stores. Reciprocally, CBS upregulations strain the urea cycle, where BH4 is needed to form nitric oxide. The formation of nitric oxide requires two BH4 molecules. With insufficient BH4, the body will instead produce peroxy nitrite (with one BH4 molecule), or super oxide (if no BH4 is available.) These two products can cause oxidative damage. The combination of CBS + and these other SNPs will further weaken this leg of the BH4 stool.

MTHFR A1298C mutations (if present) impair the second leg by disrupting the recycling and regeneration of BH4. Chronic bacterial infection (which can lead to aluminum retention) weakens the third leg of the stool, because aluminum inhibits a key enzyme that helps to synthesize BH4. On this program, you will ultimately address all three legs of the BH4 stool by supporting the body to address chronic bacteria/aluminum, supporting the MTHFR A1298C mutation, and addressing CBS/ammonia issues.

Other Interfaces that Impact BH4

Other mutations can also improve (or worsen) our BH4 stool’s sturdiness. While BH4 helps in the formation of neurotransmitters, other factors contribute to neurotransmitter breakdown. Bacterial infections trigger a more rapid breakdown of tryptophan (needed for serotonin). Low levels of BH4 have been associated with hypertension and arteriosclerosis, as well as with more severe parasitic infections. Parasitic infections also deplete B12 levels, impacting methylation cycle function.

Lack of BH4 may result in mast cell degranulation and lead to higher histamine levels, which can produce symptoms such as red ears and other hypersensitivity reactions. Serotonin synthesis as well as ammonia detoxification also require BH4. Elevated ammonia levels can cause flapping and other over-stimulatory behaviors.

Factors that lead to more ammonia, such as high protein diets, generate more ammonia that needs to be detoxified. Each molecule of ammonia requires two molecules of BH4 for ideal detoxification. Excess ammonia in the gut may alter the pH and aggravate imbalances in microbial flora. It’s obvious how these factors interact to impact ammonia detoxification as well as optimal BH4 levels for neurotransmitter synthesis. Keeping the ammonia levels under control is of paramount importance for overall health and wellness, especially for those with an MTHFR A1298C mutation, as any excess ammonia generated can drain stores of BH4. This can affect serotonin levels and to a certain extent cause fluctuations in dopamine (which translates into mood swings). Helping to restore adequate levels of BH4 should also aid in serotonin synthesis, maintaining dopamine levels as well as ammonia detoxification in a more stable manner.

Test Results Indicating Decreased BH4

• High hippuric
• Increased 8 hydroxy 2 deoxy guanosine (lack of SAMe or high ammonia can also cause increased 8 hydroxy 2 deoxy guanosine)
• Elevated phenylalanine, phenyl lactate, phenyl acetate, and/or phenylethylamine
• Increased ammonia
• Until we support the methylation cycle, we are not going to see the full impact of the CBS upregulations.

BH4 Supplementation

Preliminary collaborative research is ongoing with a group of doctors in Japan looking at the use of prescription BH4 to help to compensate for MTHFR A1298C and CBS C699T+ mutations. The initial results are encouraging. Low daily doses of BH4 (1.25 mg) initially appear to stimulate detoxification over the first several weeks of use. After this initial detoxification effect, the BH4 appears to have a very positive impact on language for individuals with CBS C699T+ mutations. It seems it is possible to restore BH4 stores through supplementation.

Ingredients: Vitamin C (as ascorbic acid) 10 mg, Tetrahydrobiopterin (a stable form of the physiologic agent, biopterin) 2.5 mg. Other ingredients: Sucrose, Gelatin, L-Leucine.